aarch64: morello: fix missing variadic argument in fcntl

aarch64: morello: fix missing variadic argument in fcntl

In fcntl va_arg is currently used even if the caller did not pass
any variadic arguments. This is undefined behaviour and does not
work with the Morello purecap ABI, so use a helper macro.

When the argument is missing, the result of the helper macro is
arbitrary as it will be ignored by the kernel, we just have to
ensure it does not cause a runtime crash.